Method and apparatus for use in determining prothrombin time of a blood sample



y 1970 s. STEINBERG E AL 3,520,659

METHOD AND APPARATUS FOR USE IN DETERMINING PROTHROMBIN TIME OF A BLOOD SAMPLE 2 Sheets-Sheet 1 Filed Oct. 20, 1967 EL 50 TRON/6% T/MER FIG. 4 INVENTORS gHELDON $VTI'1EjrNEBERG BY J SESH E VAN AL A ORNEVS United States Patent METHOD AND APPARATUS FOR USE IN DETER- MINING PROTHROMBIN TIME OF A BLOOD SAMPLE Sheldon Steinberg, Woodland Hills, Serge DeWitte, Los Angeles, and Josephus van Balgooy, Durate, Calif., assignors to Xerox Corporation, Rochester, N.Y., a corporation of New York Filed Oct. 20, 1967, Ser. No. 676,820 Int. Cl. G01n 11/10, 33/16; H02k 1/22 U.S. Cl. 23230 16 Claims ABSTRACT OF THE DISCLOSURE A method and apparatus for use in determining prothrombin time wherein solidification of the blood plasma is detected electromagnetically. Holding means having the blood plasma-clotting agent mixture in contact with a rotatable magnetic member is positioned over an external rotating magnetic member having means disposed thereabout for sensing an alteration in the adjacent magnetic field due to the solidification of the sample material. A current pulse generated within said sensing means concomitantly with solidification is utilized to deactuate timer means.

BACKGROUND OF THE INVENTION This invention relates to automatic chemical analysis and, more particularly, the invention relates to a method and apparatus for determining the prothrombin time of blood plasma. While the invention will be described primarily with respect to prothrombin time determination, it should be understood that the present invention is also applicable to other forms of analysis, egg. in the determination of the coagulation properties of whole blood. In essence, the present invention is particularly suitable for accurately determining a particular end point evidenced by a rapid increase in the viscosity of the material being analyzed.

Blood clotting or coagulation is the transformation of liquid blood into a semisolid, gel-like state of consistency. Clotting is essentially a function of the plasma and involves the changing of one of the plasma proteins, fibrinogen, from the sol(liquid) to the gel(solid) state. This change in consistency is brought about by the presence of tiny, thread-like, insoluble structures which form an interlacing network of fibers made of a protein called fibrin.

The clotting processes are initiated and accelerated by the juices from injured tissues. Both injured tissues and. disintegrated blood platelets give off similar substances, collectively thrombokinase, which initiate the clotting reaction. When blood escapes from an injured vessel, it is immediately exposed to the juices of damaged tissues thereby automatically starting the clotting process.

An immediate factor in the clotting mechanism is thrombin, because it acts to change fibrinogen into fibrin and thus produce a clot. Under medical theory as presently believed accurate, thrombin exists in the blood in an inactivate form, prothrombin, which is changed by thrombokinase and calcium into thrombin at the time of clotting. Prothrombin itself does not occur as such in normal plasma, but is combined with a substance called antiprothrombin which with thrombokinase is united in order to initiate the clotting mechanism. This union releases prothrombin. This prothrombin is changed into thrombin in the pressence of calcium. The thrombin, together with fibrinogen, the soluble protein present in normal plasma, produces fibrin, the insoluble protein which constitutes the actual clot.

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The prothrombin time test is a measure of the clotting time of plasma to which a tissue thromboplastin suspension has been added. That is, prothrombin time is the time required for the appearance of a clot in a sample of blood plasma treated with a coagulant, such as thromboplastin, in the presence of calcium. Normal prothrombin times range from about 11 to about 15 seconds and are relatively constant in man. Any deviation from the normal prothrombin time is indicative of either a latent abnormality in the physical health of the patient or of a faculty analytical technique.

The prothrombin time determination is extremely beneficial in categorizing the bleeding states due to coagulation defects. Additionally, the recent development and wide spread use of anti-coagulants requires that extreme care must be exercised because the response of different patients to different drugs and different dosage levels of drugs varies significantly. To maintain the anti-coagulant dosage at a proper level thereby reducing the possibility of further damage, minor or fatal, to the patient, the prothrombin time must be accurately determined. Only if the physician has reliable data can he knowingly prescribe, with little fear of adverse affects, the proper dosage level.

A common characteristic of prothrombin time deterniinations is that a patients plasma is added to a thromboplastin-calcium mixture and the time elapsed for the formation of fibrin or a clot is measured. In a typical procedure, fresh blood is drawn, mixed with a measured portion of anti-coagulant, and then centrifuged to separate the blood plasma from the cells. The plasma is then placed in a test tube which is maintained at approximately 37 C. Thereafter, but before damage to the plasma, a measured portion of thromboplastin and calcium chloride is added thereto. Timing compensates with the addition of this latter mixture and terminates with the formation of the clot. Where this procedure is run manually, it is subject to all the deficiencies normally inherent in manual procedures of all types. Visual determination of the end point in clotting is viewed differently by different technicians since the manifestations thereof can be and usually are non-uniform. The judgment of the technician is basically subjective, and tends to vary with fatigue, emotion, and minor distractions. Additionally, ambient environmental conditions, such as lighting, etc., affect the technicians judgment thereby resulting in different findings. The necessity of performing many functions, such as the thorough mixing of the plasma and reagents and the simultaneous starting of the timer, add to the difiiculty of achieving reliable data.

OBJECTS OF THE INVENTION It is an object of the present invention to provide an automatic apparatus for accurately determining prothrombin times and measuring the coagulation properties of blood.

It is an object of the present invention to provide an apparatus for determining prothrombin times and for measuring the coagulation properties of blood which automatically signals the end point of the analysis.

It is an object of the present invention to provide an automatic apparatus for determining prothrombin times and for measuring the coagulation properties of blood which does not require the operator to determine when the end point has been reached.

It is a further object of the present invention to provide an automatic analytical apparatus for electromagnetically determining prothrombin times and measuring the coagulation properties of blood.

Still a further object of the present invention is to provide an automatic analytical apparatus for electro- 3 magnetically determining prothrombin times and measuring the coagulation properties of blood which includes means to dispense prepackaged reagents from their storage compartments into the liquid sample material present in a lower compartment of said disposable reaction container.

Still a further object of the present invention is to provide a novel electromagnetic method for determining prothrombin times and the coagulation properties of blood.

Still a further object of the present invention is to provide a novel method for running prothrombin time determinations and coagulation property determinations wherein the necessity of having the technician add reagents to the reaction vessel is eliminated.

The above and still further objects, features, and advantages of the present invention will be apparent upon a consideration of the following detailed disclosure of specific exemplary embodiments of the invention.

SUMMARY OF THE INVENTION The above and still further objects of the present invention are achieved by providing an automatic analytical system having, in its essential elements, means to support a container or other means for holding a measured quantity of liquid sample material and a rotatable magnetic member therein, means external to the holding means and magnetically coupled to the rotatable magnetic member for horizontally rotating the member at a constant angular velocity through the liquid sample material, and means electromagnetically coupled to the rotating member for sensing an alteration in the angular velocity of the magnetic member Within the holding means due to an increase in viscosity of the liquid sample material.

The end point of the analysis is sensed by first establishing the movement of the rotatable magnetic member at a constant angular velocity through the liquid sample material and thereafter detecting a change from the steady state angular velocity in response to an increase in the viscosity of the liquid sample material. This sensing is achieved by providing magnetic means for urging the magnetic member to rotate through the liquid sample material and disposing, about said magnetic urging means, coils which, when the rotation of the magnetic member is altered, will have a pulse generated therein due to the modification of the existing magnetic field. This pulse will signify the end of the determination and can be utilized to deactuate the timer mechanism.

BRIEF DESCRIPTION OF THE DRAWINGS The nature of the invention will more easily be understood when it is considered in conjunction with the accompanying drawings wherein:

FIG. 1 is a top view of a container in which the analysis is to be conducted;

FIG. 2 is an end view of the container of FIG. 1;

FIG. 3 is a flow chart illustrating the procedural steps of the present invention and the apparatus therefor; and

FIG. 4 is a combined schematic and section view of portion of the apparatus of this invention.

In a preferred embodiment, the apparatus includes a holding means in the form of a disposable reaction container having an upper section for the storage of prepackaged reagents, such as thromboplastin and calcium chloride, and a lower reaction compartment. Disposed in the lower reaction compartment is a freely movable ferromagnetic member. Restraining means in the form of a thin layer of sufficient strength is provided to prevent premature movement of the prepackaged reagents from their storage chambers. Disposable reaction containers suitable for use in the present invention are more fully described in co-pending applications Ser. Nos. 602,018 and 602,025 filed Dec. 15, 1966 and Ser. No. 645,665 filed June 13, 1967, now abandoned, all of said co-pending applications being assigned to the assignee of the present invention. Portions of those applications which are necessary to a complete understanding of the present invention are incorporated herein by reference. It should be understood, however, that other containers are suitable for use in the practice of the present invention. The aforementioned reaction containers are preferred because they can be disposed of after analysis and provide a most convenient means for storing and transporting necessary reagents, if any, to the material under analysis.

An exemplary disposable container suitable for use in the practice of the present invention is shown in FIGS. 1 and 2 wherein package 10 has a lower section 12, an upper section 14 and a restraining layer 16 for maintaining stored reagents in the plurality of reagent storage chambers 18, 20, etc. Lower section 12 has a lower compartment 22. Disposed in the lower compartment is a freely movable ferromagnetic stirring bar 24. Resting on lower section 12 is an upper storage section 14 which comprises an upper layer 26 defining a plurality of reagent storage chambers 18, 20, etc. in the form of top-hats. On the underneath or open portion of layer 26 is a thin, weak restraining layer 16 for holding the reagents in the respective chambers. Application of force on the top of the chambers will cause a shearing of restraining layer 16 at a point immediately below the top-hat resulting in the inversion thereof. Reagents and/ or other materials stored therein will be emptied into the lower compartment. This is diagrammatically shown in FIG. 3 of co-pcnding application Ser. No. 602,018. Upper section 14 has a fiange 28 extending around the perimeter of the plurality of reagent storage chambers. One side of this flange which extends the length of the disposable reaction container is slightly wider than corresponding portions which encircle the remainder of the upper storage section 14. This wider portion is indicated at 30. Lower section 12 has a (flange 32 which encircles the upper perimeter of the lower section. Flange 32, as well as restraining layer 16, is also wider along those portions beneath the wider portion 30 of flange 28. Thus, the rectangles with slightly rounded edges formed by flange 32 encircling the upper perimeter of lower section 12, restraining layer 16, and flange 28 encircling the lower perimeter of upper section 14 are of equal size and dimension so that the three members can be suitably joined to provide a unitary disposable container. Preferably, each member is formed out of a plastic material which can be heat sealed to the adjacent member or members to provide an exceptionally strong bond which cannot be broken under normal use. Flanges 28 and 32 and restraining layer 16 are sufiiciently Wide along the wider portion 30 so that a code area 34 can be provided between the inner bond 36 and outer bond 38. Any suitable type of coding can be placed on this coded area to indicate or record any information which should be known during a chemical analysis, such as the actual test which has been pre-stored in the particular disposable reaction container, the patient number, instructions for the associated automatic analytical apparatus and system, analytical results, etc. Typical codes include binary coding in the form of light and dark areas, magnetic coding, etc.

Referring to FIG. 3, the holding means or container is stored in storage means 50 from which it is ejected by container ejector 52 and placed onto transportation means (not shown) for movement through the analytical system. Stored reagents which may be in either liquid or tablet form and a metered quantity of diluent, such as water, are added to the container at station 54 and, if necessary to bring the reagents into solution, can be mixed as designated at 56- by suitable mixing means. An exemplary mixing means is a rotating magnetic bar for urging the rotatable magnetic member within the disposable container to revolve through the reagent solution. Ultrasonic mixing is also applicable. Further, if necessary for the particular analysis, the reagent solution can be heated above room temperature as designated at 58 by any suitable heating means. In the case of blood analysis, the reaction solution is brought to 37 C. (i.e. body temperature) to more closely approximate normal coagulation conditions. In conducting other analyses, the temperature to which the reagent solution is heated can be selected as desired or as determined by standard analytical techniques. A metered quantity of liquid sample material is added at sample addition station 60, the addition thereof automatically starting timer 62. If necessary, further mixing and heating means can be positioned adjacent the liquid sample-reagent solution holding container for providing the desired ambient analytical conditions. Reagent and water addition, mixing, heating, sample addi tion, etc. can be provided in a single station or in a plurality of stations as desired or as determined by the particular analytical technique. Suitable and exemplary apparatus for storing the containers, and for adding the reagents and liquid sample material to the disposable container are more fully described in the aforementioned copending applications Ser. Nos. 602,018 and 602,025 filed Dec. 15, 1966. Reference may be had thereto for said complete discussion. It should be understood, however, that other means, obviously suitable for the same purpose, may be utilized to achieve the same result. For example, injection means can be provided for the addition of liquid material to the container, said means including a hollow injection needle for penetrating upper storage section 14 and restraining layer 16. The material is added to the lower compartment through the interior of the needle which is then withdrawn. A further exemplary aliquotter is the one shown in Ser. No. 602,080 filed Dec. 15, 1966 and assigned to the assignee of the present invention. The automatic aliquotter and the analytical apparatus as shown in said latter co-pending application are incorporated herein by reference. The aliquotter is positioned adjacent the sample carrier, such as the storagesite-bearing tape of the latter mentioned application or a circular sample wheel as disclosed by Skeggs in U.S. Pat. No. 2,879,141. The aliquotter withdraws a measured portion of sample material from one of the storage cavities and transfers said measured portion to the disposable container. As shown in said applications, the aliquotter has a measuring tube which is inserted into the sample material and operatively connected to a source of vacuum which draws a predetermined amount of liquid sample into the measuring tube. When the tube is filled, a drop is formed on the interior end thereof. This drop is sensed by detection means comprising a lamp and a photocell. The aliquotter is caused to oscillate until the tip of the measuring tube is positioned directly over the point at which the fluid is to be emptied from the tube. Slight pressure is placed on the interior end of the measuring tube forcing the liquid contents thereof out of the tube and into the container. If necessary, means can be provided either before or at the sample addition station for punching a hole in the disposable container so that the liquid material can be added through the upper section to the lower compartment.

The analysis proceeds until the viscosity of the liquid sample material has increased to a point where the rotation of the magnetic stirring bar disposed within the reaction compartment of the disposable container is prevented from rotating or, at least, hindered in its rotation. At this point in time, as generally indicated at 64, a signal pulse is generated, for example by the apparatus disclosed in FIG. 4, which can be utilized to stop the timer mechanism, as generally indicated at 66. The start and stop signals are correlated and stored in data storage device 68 from which the analytical data can be read out by data readout device 70 when desired. After the analysis is completed, the container having the viscous reaction mixture therein is ejected from the end-point determination zone 64 (which can be, and normally is, sample addition zone 60) by container ejecter 72 and sent to disposal or storage.

As will be apparent from the foregoing discussion, the

entire analysis is conducted automatically without requiring the laboratory technician to perform any manual operations other than (1) the actual taking of the sample material and (2) the operations required to initiate the operation of the described apparatus. The necessary reagents for running the particular analysis are stored within the packagethere is no need to carefully measure and dispense precise quantities of liquid reagent material. Sample addition is accomplished automatically and precisely so that the technician need only take the sample material and place it in a particular storage cavity from which a metered quantity will be withdrawn. Detection starts with the addition of the sample material and is terminated automatically when a particular end point has been reached. The analysis is thus made independent of inherent human variations in selecting, by pu'rely physical observations, the desired end point. Accordingly, as will be shown in the ensuing example section, the apparatus yields highly reliable and reproducible data having a standard deviation well within the tolerance limits for the particular analysis herein described.

Referring to FIG. 4, there is shown a disposable reaction container 10 as it is positioned within read-out means 80. The top-hats (not shown) have already been inverted whereby their stored reagents have been deposited in the lower compartment as indicated in FIGS. 1 and 2. Additionally, metered quantities of water and liquid sample material have been added to the lower compartment. Simultaneous with the addition of the liquid sample material, the timer 81 is started to signify the beginning of the analysis. Disposable container 0 is supported by means 82 upon which it is brought into the read-out means and upon which it will be ejected therefrom. The interior of read-out means comprises an aluminum channel 84 sufficiently wide to permit the passage of disposable container 10 therethrough.

Disposed on the outside of channel 84 is a channel 86 of magnetic insulation material for isolating the analytical zone from unwanted magnetic fields. Channel 86 is attached to plate 88 which in turn is connected by spring loaded adjusting screws 90 to a second plate 92 positioned on the top of motor 94. Armature 96 extends out of motor 94 through plate 92 and into an annular aperture 98 in plate 88, insulation 86 and aluminum channel 84. Disposed about armature 96 and positioned directly beneath disposable container 10 is a cylindrical magnet 100 having a cylindrical steel rod 102 soldered on the upper surface thereof. Disposed about steel rod 102 is a wire coil 104 having one end connected to slip ring 106 and the other end to slip ring 108. Leads 110 connect the slip-ring assembly to the electronics 83 for translating the alternating current pulse generated by the modification of the magnetic field at the end point of the determination into a stop signal for the timer 81.

In operation, the read-out mechanism of FIG. 4 including magnet 100, steel rod 102 attached thereto, and coils 104 is caused to rotate at a fixed angular velocity. This causes the ferromagnetic stirring bar 24 disposed within the lower compartment of container 10 also to be rotated at a constant angular velocity through the liquid mixture residing therein. With regard to blood property determinations, upon formation of the clot the rotating ferromagnetic stirring bar 24 is held tightly thereby and kept from rotating or, at least, hindered in its rotation. This change in the rotation of the ferromagnetic stirring bar 24 induces an alternating current to be generated in the coil 104 which are electromagnetically coupled to the cylindrical magnet 100. This signal is carried from the coil 104 through the slip ring assembly and the leads 110 attached thereto to stop the timer 81. Since the beginning and end points of the determination are not dependent upon human observations or actions, the herein disclosed apparatus is capable of providing exceptionally reliable and accurate data well within the tolerance limits established for the particular analysis.

7 DESCRIPTION OF SPECIFIC EMBODIMENTS The following examples are given to enable those skilled in the art to more clearly understand and practice the invention. They should not be considered as a limitation upon the scope of the invention but merely as being illustrative thereof.

The following embodiment of a prothrombin time determination will be described with reference to the preferred holding means as exemplified by the disposable container of FIGS. 1 and 2. At a reagent addition station, the thromboplastin and calcium chloride are forced from their respective storage chambers into the lower compartment by the application of force to the top of each storage chamber causing rupture of the restraining layer and release of the reagents therefrom. The disposable container is advanced by the transportation means to a position beneath a punch which punches a minute hole through the upper storage section and restraining layer. The container is advanced further to a water addition station wherein a metered quantity of water is added through the previously punched hole. Simultaneously therewith, a magnet situated below the disposable container is rotated causing rotation of the rotatable magnetic member within the disposable container thus bringing the thromboplastin and calcium chloride into solution. As the disposable container is indexed forward, heating means external thereto are provided or raising the temperature of the reagent solution to approximately 37 C. and or maintaining it at that level. At the analytical station, a metered quantity of plasma specimen is added to the lower compartment and simultaneously therewith the timer is started. The rotating magnet in the analytical station causes the rotating magnetic member within the disposable container to mix the plasma with the reagent solution and it continues to rotate at a constant angular velocity until the increased viscosity of the total liquid material alters the rate of revolution. Upon formation of the clot, the

magnetic member is held tightly thereby and kept from rotating. This alters the magnetic field and induces an alternating current in the coil surrounding the still rotating magnet which is automatically converted into a stop signal for the timer mechanism.

EXAMPLES I-XXXVIII A series of prothrombin time determinations were performed on the apparatus of the present invention and a presently available commercial unit. The prothrombin timer of the present invention including a 90 rpm. synchronous motor having 411 inch ounces of torque when operated at 60 cycles. There are 2 coils wired in series about the rotating magnet, each coil having 8,000 turns of No. 50SSG wire offering 3,000 ohms resistance, giving a total resistance of 6,000 ohms. In this series of tests, the and normal plasma control (Hyland Lot No. 357B003A) and normal human plasma control (Hyland Lot No. 04710D16A1) were used on both machines. The purpose of these tests was to determine the reproducibility of the determinations on each machine for a specific set of conditions.

Both machines were allowed to come to 37 C. for approximately 25 minutes before the tests were initiated. The determinations made on the apparatus of the present invention were made in a single disposable container which was kept in a fixed position over the magnetic readout mechanism. Although the same disposable container was used for the determinations, several different Tefloncoated stir bars were used during the study. With the apparatus of the present invention the plasma samples were added automatically to the thromboplastin solution in the disposable container. The plasma samples were manually added by the operator to the commercial unit.

In the initial set of data on the commercial unit there appeared to be a wide variation in the values obtained which was, at first, throught to be due to operator technique. This first set of data is represented by Examples 1- 8 VIII. A second set of data, therefore, was obtained by another operator immediately after the first set and is represented by Examples IX-XX. Examples XXI- XXXVIII represent the determinations made with the apparatus of the present invention. The results in seconds are given below in the table.

Standard deviations given in the following table for each of the sets of data and the combined commercial unit data were calculated according to the formula:

0' is the standard deviation X is the sample value if is the average value n is the number of samples TAB LE where Commercial Unit Apparatus herein Average=12. 133 4T= 0. 450

Combined average=12.150

Averagc= 12. 166 103 The results indicate that the apparatus of the present invention under identical conditions yields considerably more reproducible data than the prior art device. Although the average prothrombin time obtained from both machine is practically the same, the standard deviation of the apparatus of the present invention is less than A that of the prior art device. Further, the standard deviation is well within accepted limits for this particular determination.

It should be noted that the number of coils, wire turnings, and resistance values need not be fixed to the items or values specified. These items are variable and depend upon the degree of sensitivity and noise to signal ratio desired. Additionally, as will be apparent to one skilled in the art, the geometry of the system may also be varied. For example, rather than having the magnet, coils, and iron cores in the coils all moving at the same time and speed, it is feasible to have only the magnet and iron cores moving and to have the coils stationary. This eliminates the need for slip rings since the stationary coils can be directly wired to an amplifier or other electronic circuits. As a further example, a mercury contact system can be substituted for the slip rings for transmission of current changes. Thus, while the invention has been described with reference to referred embodiments thereof, it should be understood by those skilled in the art that various changes in form and details may be made without departing from the true spirit and scope of the invention. All such modifications, etc., are considered to be within the scope of the present invention as defined by the claims appended hereto.

What is claimed is:

1. The method of measuring the prothrombin time of blood plasma comprising mixing an aliquot of blood plasma with a clotting agent, horizontally rotating a magnetic stirring bar at constant angular velocity through the combined liquid mixture, and electromagnetically detecting an alteration in the angular velocity of said rotating magnetic bar due to the solidification of the blood plasma.

2. The method of claim 1 wherein the clotting agent comprises thromboplastin and calcium chloride.

3. The method of claim 2 wherein the thromboplastin and the calcium chloride are each added in tablet form. A

4. A prothrombin timer comprising means to support holding means having a movable magnetic member in a liquid sample material holding compartment, means external to the holding means including means magnetically coupled to the movable magnetic member for causing relative movement of the holding means and the magnetic member in liquid sample material therein, timer means, and means for sensing a detectable change in the relative movement of the magnetic member with respect to the holding means due to the solidification of the liquid sample material within the holding means, said sensing means having a current pulse generated therein in response to said solidification which is utilized to deactuate said timer means.

5. A prothrombin timer comprising means to support holding means having a movable magnetic member in a liquid sample material holding compartment, means external to the holding means including means magnetically coupled to the movable magnetic member for causing relative movement of the holding means and the magnetic member in liquid sample material therein, timer means, means to add liquid material to the holding means, said timer means being actuated by the addition of said liquid material, and means for sensing a detectable change in the relative movement of the magnetic member with respect to the holding means due to the solidification of the liquid sample material within the holding means, said sensing means having a current pulse generated therein in response to said solidification which is utilized to deactuate said timer means.

6. The prothrombin timer of claim 5 wherein said means to add liquid material to the holding means adds liquid sample material thereto, said timer means being actuated by the addition of said sample material.

7. Apparatus for use in determining the prothrombin time of a blood sample comprising: a container for holding the blood sample, a freely movable ferromagnetic member disposed in the container, a magnetic field source located near the container and forming a weak magnetic couple with the ferromagnetic member, means for producing relative movement between the ferromagnetic member and the container, the intensity of the magnetic field source being sufficient to prevent relative movement between the ferromagnetic member and the magnetic field source prior to clotting but insufficient to prevent relative movement between the ferromagnetic member and the magnetic field source on clotting due to an increase in the resistance to movement of the ferromagnetic mem her in the container imposed on the ferromagnetic member by the blood sample, and means for deriving a signal in response to relative movement between the ferromagnetic member and the magnetic field source, the signal signifying the end point in determining the prothrombin time. 1

8. The invention according to claim 7 and wherein the ferromagnetic member is a bar totally unsupported in the container.

9. The apparatus according to claim 7 and wherein the means for producing relative movement between the ferromganetic member and the container comprises a motor connected to the magnetic field source for moving the magnetic field source relative to the container, the magnetic couple between the ferromagnetic member and the magnetic field source causing corresponding relative movement of the ferromagnetic member.

10. The apparatus according to claim 7 and wherein the magnetic field source includes a magnet.

11. The apparatus according to claim 10 and wherein the magnet is located adjacent the container.

12. The apparatus according to claim 11 and wherein the magnet is located underneath the container.

13. The apparatus according to claim 7 and wherein the means for deriving a signal indicating the relative movement between the ferromagnetic member and the magnetic field source comprises a coil positioned so that the relative movement will generate therein a detectable electrical signal.

14. The apparatus according to claim 13 and wherein the coil is mounted in fixed relationship relative to the magnetic member and wherein the electrical signal for indicating the relative movement is an AC. signal induced in the coil.

15. The apparatus according to claim 13 and further including timing means electrically connected to the coil and deactuable in response to the detectable electrical signal generated in the coil.

16. The apparatus according to claim 15 and further including magnetic insulating means surrounding the container and magnetic member for insulating the container and magnetic member from external magnetic forces.

References Cited UNITED STATES PATENTS 2,879,141 3/1959 Skeggs 23253 3,053,078 9/1962 Jewett 7354 3,162,038 12/1964 Roberson et a1 7359 MORRIS O. WOLK, Primary Examiner R. M. REESE, Assistant Examiner US. Cl. X.R. 

